In patients with normal kidney function, calcium and phosphorus balance is maintained through the interaction of parathyroid hormone (PTH) and calcitriol, an active metabolite of vitamin D. PTH provides a mechanism for controlling extracellular calcium and phosphate concentrations by regulating intestinal reabsorption, renal excretion, and exchange of these ions between the extracellular fluid and bone.
With progressive renal insufficiency, however, there is increased phosphorus retention by the failing kidney. In order to restore phosphorus balance, compensatory elevation in PTH levels is triggered, which increases renal resorption of calcium, while decreasing tubular resorption of phosphorus. The net effect of compensatory hyperparathyroidism in this early stage of renal disease is that serum phosphorus levels are maintained within the normal range.
The retention of phosphorus as a result of the decreased ability of the diseased kidney to excrete the filtered phosphate leads to a decrease in serum free calcium, which in turn stimulates the secretion of more PTH. With each progressive reduction in kidney function, a new steady state is achieved in which serum phosphate is restored to normal at the expense of a sustained high level of PTH. The cycle is repeated as renal function declines until sustained and severe hyperparathyroidism is present; eventually the compensatory mechanism is not able to control the increasing serum phosphorus levels. Once the glomerular filtration rate has decreased to <20% of normal, overt hyperphosphatemia becomes evident. In end-stage renal disease patients (where the compensatory mechanism mediated by PTH is no longer effective), the increase in plasma phosphate results not only from decreased excretion but also from continual high levels of PTH, which further exacerbates the problem by releasing calcium and phosphate from the bone.
The clinical manifestations of hyperphosphatemia are varied and have considerable mortality risks. Severe hyperphosphatemia can induce hypocalcemia, which aggravates the imbalance in PTH levels further by increasing the production of this hormone. Hyperphosphatemia inhibits renal synthesis of calcitriol, which causes an exacerbation of the hypocalcemia condition. The occurrence of severe hypocalcemia with tetany and ectopic calcifications is the most severe manifestation of hyperphosphatemia. Calcification may occur in the joints, soft tissues, lungs, kidney, and conjunctiva. Soft tissue calcification has been linked to cardiovascular risk, and cardiovascular disease is the cause of death in more than 45% of all dialysis patients. Renal osteodystrophy with effects on the bones and muscles is common in end stage renal disease (ESRD) patients, as well as severe pruritis. The high PTH level associated with developing and severe renal disease has indirect actions on the central and peripheral nervous system, and the myocardial tissues, creating further disorders such as hyperlipemia, muscle growth retardation, arteriosclerosis, bone loss, and immunodeficiency.
Prevention and treatment of hyperphosphatemia is achieved by a variety of means, including dietary control of phosphorus intake, dialysis and oral phosphate binders. Dialysis, however, does not remove phosphate ions well from the serum because of the slow equilibrium between intracellular and extracellular phosphorus. The treatments of choice focus instead on a phosphorus controlled diet and the administration of phosphate binders taken at meals. A low phosphorus diet is not a long-term option, however, since patient compliance is difficult and the daily dietary phosphorus intake cannot be lowered below ˜1000 mg/day, without restricting protein intake even further than the 1.2 g/kg/day of protein recommended for hemodialysis patients.
Oral phosphate binders comprise two main classes: inorganic metal salts and polymer resins, often referred to as metal-free binders. Examples of the former category include compounds such as aluminum carbonate, calcium carbonate, calcium acetate (PhosLo), and lanthanum carbonate (Fosrenol). While aluminum and calcium salts have been the treatment of choice for years, they produce soluble metal ions that cross the gastrointestinal membrane and enter the blood stream, producing toxic effects. For instance, aluminum carbonate salts have been shown to be involved in cases of encephalopathy and aluminum osteopathy due to aluminum bone absorption. Calcium binders also generate large amounts of soluble calcium cations, the absorption of which can cause hypercalcemia. Further, although the causative effect is not fully demonstrated, high calcium x phosphate product has been held responsible for soft tissue calcification and cardiovascular disease. Lanthanum carbonate seems to produce less metal absorption, but bone accumulation of lanthanum has been established and the long-term effect of such accumulation in humans is still unclear.
Metal free binders include ion exchange resins and crosslinked polyallylamine resins. Ion exchange resins include cholestyramine, colestipol hydrochloride, and Dowex. These resins have been proposed as an alternative to metal salts, but their low capacity and their lack of palatability have precluded their wide use in the clinic. Crosslinked polyallylamine, like sevelamer hydrochloride (Renagel), was introduced as the next generation of metal-free phosphate binder resins. However, the phase 1 clinical trials performed on healthy volunteers indicate that the in vivo binding capacity of Renagel is much lower than anticipated from in vitro studies. As a consequence ESRD patients still need a high dosage of Renagel to meet clinical end-points, leading to adverse effect such as gastrointestinal discomfort and problems with patient compliance.
Accordingly, there is a need to develop better phosphate binding therapies, with reduced side effects for patients with hyperphosphatemia.